This invention relates to a thermally activated electrochemical cell. In a more particular aspect, this invention concerns itself with a high energy density, expendable, primary thermal battery in pelletized form which utilizes aluminum chloride containing electrolytes.
The present interest in the development of weight sensitive communication systems for both military and civil applications has generated a considerable research effort in the fabrication of light weight, maintenance free, high density power supplies. The emphasis in the research effort has been placed on those experimental investigations and theoretical studies which elucidate the nature of the AlCl.sub.3 /NaCl electrolyte system and the behavior of electrodes immersed in these electrolytes.
For example, it has been found that, if a concentration cell was created using two aluminum electrodes and two different AlCl.sub.3 /NaCl electrolytes, a significant voltage of about 0.2 V could be achieved. Also, it was found that, if one of the electrolytes in the concentration cell was held saturated with respect to sodium chloride, a markedly enhanced voltage of about 0.6 V could be obtained. Additional research resulted in the fabrication of an aluminum-chlorine formation cell which used a paste cathode consisting of a mixture of electrolyte and graphite powder. These cells were assembled in large test tubes with corks in the tops. In spite of the crude construction, the cells had very promising features. Perhaps the most noteworthy feature was the ability to deliver large currents without apparent polarization or passivation. However, implementation of these cells was not found feasible for practical applications. Additional research was necessary, therefore, in order to fabricate a workable and practical battery capable of taking advantage of the attractive features of the aluminum-chlorine electrochemical system. The thermal battery of this invention resulted from that research effort and provides a practical solution to the problem of providing a light-weight, reliable, maintenance free power source operable at temperatures ranging from 109.degree. to 222.degree. C.